To modify the surface of metal materials, for example, a plated steel sheet, a steel sheet is treated with various coating compositions and surface treating chemicals.
The object of the treatment is to improve the prevention of leaving fingerprints, workability, chemical solution resistance, and corrosion resistance, and these properties are improved by mainly applying an organic resin on the surface of a steel sheet.
For the case in which the metal material is worked after being subjected to a surface treatment, a soft organic resin having good workability is often used to impart appropriate workability. When using such a soft organic resin, it frequently becomes difficult to impart properties such as surface hardness, solvent resistance, and wear resistance.
Also, treatments for forming an inorganic coating film, for example, a chromate treatment and zinc phosphating treatment, are commonly conducted. These treatments impart good corrosion resistance and solvent resistance, but the workability is inferior as compared with the case of using an organic resin.
For the case in which the metal material is coated with an organic coating film, the organic coating film layer is often formed after forming an undercoat layer by a chromate treatment so as to enhance the adhesion between the metal surface and the organic coating film layer. However, it is reported that hexad chrome contained in a chromating agent and a coating film is toxic to organisms and is an environmental burden, and thus a treating method is required using a treating agent having low toxicity and a small environmental burden, which replaces the chromate treatment.
However, it is difficult to develop a treating agent, which has low toxicity and a small environmental burden and which can ensure an adhesion equivalent to that for the case of the chromate treatment. A chemical for a coating treatment of the metal surface which is capable of imparting sufficient adhesion could not be obtained. Therefore, a treating chemical and a coating layer, which exhibit good adhesion, are highly safe, and are highly sanitary with or without chromate treatment, are now required.
In the field of the working of metal materials, particularly metal sheets such as aluminum, copper, and iron sheets, or plated steel sheets, the metal sheets have hitherto been worked after applying a lubricating oil to the surface thereof, and are then degreased.
The steps of applying a lubricating oil only for working, followed by working and further degreasing are complicated, so that it is strongly desired to simplify the step of applying a lubricating oil and the step of degreasing. Therefore, development of a coating composition for forming an organic coating film has been strongly desired which has a surface lubricating function and a rust preventing function or a function capable of wet-on-wet coating of another coating composition, and also has high adhesion even without being subjected to a chromate treatment.
To obtain a viscosity of the coating composition which is suitable for coating, such as coating and printing, a solvent type coating composition to be diluted with a solvent has heretofore been widely used in conventional coating compositions, coating agents, and inks. The use of an organic solvent has problems in aspects of safety and health as well as the working environment.
For the case of a solvent-free type coating composition, although a drying step is not required, heating of the coating composition is sometimes required to obtain a viscosity of the coating composition which is suitable for coating because the coating viscosity is too high at normal temperatures. Furthermore, when the viscosity is lowered by adding a monomer, a problem remains in that a diluting monomer to be added is toxic to the skin and has an odor, which is similar to the problem of the solvent type coating composition.
Moreover, when using a high-molecular weight binder resin to improve the physical properties of the coating film, the viscosity of the coating composition becomes too high, thereby making it impossible to use such a high-molecular weight binder resin. Therefore, it is difficult to obtain excellent physical properties of the coating film.
To solve these problems, intensive studies and developments about an aqueous coating composition, in which the viscosity of the coating composition can be easily adjusted using water which is safe and nontoxic, have recently been made.
Dispersion or dissolution of a resin in an aqueous medium is attained by using a dispersant or an emulsifier, or a resin which has a polar group on a main or side chain and also has solubility or self-dispersibility in water.
The latter self-dispersible or water-soluble resin has guaranteed comparatively excellent properties. However, when using a resin having a polar group in a molecule as a binder resin of the coating composition, the resulting coating film tends to be inferior in water resistance and solvent resistance.
As a means which can completely solve these problems, for example, problems about safety and the environment, while maintaining the required physical properties of the coating film, aqueous radiation curable coating compositions, coating agents and inks have been proposed.
Japanese Unexamined Patent Application, First Publication No. Hei 8-259888 proposes an aqueous dispersion comprising water and radiation curable microgel particles dispersed in water.
The microgel particles are gel-like film-forming polyurethane resin particles having a crosslinked structure obtained by crosslinking polyurethane resins through a urethane bond or a urea bond, and the polyurethane resin has a radiation curable ethylenically unsaturated double bond and a salt group.
The above publication describes that the coating film-forming polyurethane resin particles can contain 1 to 50% of another radiation curable compound and an initiator for the radiation curing reaction therein, and that an aqueous coating film having excellent water resistance and excellent solvent resistance can be obtained by using these radiation curable microgel particles regardless of the aqueous coating composition.
Japanese Unexamined Patent Application, First Publication No. Hei 9-31150 discloses, as a composition capable of forming a highly crosslinked coating film, a radiation curable aqueous polyurethane resin composition prepared by reacting a product, which is obtained by the Michael addition of a hydroxyl compound having an active hydrogen to a double bond of a compound having two or more (meth)acryloyl groups, a polyhydroxy compound having an acidic group, and a polyisocyanate as an indispensable component.
Generally, a composition having an acryloyl group, which is superior in curability, has conventionally been used as a radiation curable composition, and Japanese Unexamined Patent Application, First Publication No. Hei 10-251360 discloses that a radiation curable aqueous polyurethane composition having a methacryloyl group is superior in adhesion, chemical resistance, surface hardness, and yellowing resistance, and is used in coating compositions, coating agents, and inks.
Japanese Unexamined Patent Application, First Publication No. Hei 10-251361 describes that excellent adhesion, excellent solvent resistance, excellent workability, and excellent coatability can be obtained by mixing radiation curable microgel particles with a radiation curable aqueous polyurethane resin.
These radiation curable aqueous polyurethane resin compositions are used in coating compositions, coating agents, and inks because of their excellent adhesion to any of organic materials (e.g., plastic films, wood construction materials and decorative papers for construction materials) and inorganic materials (e.g., metal materials and glass) and because of their excellent solvent resistance.
Any of these compositions is a radiation curable aqueous composition containing an aqueous polyurethane resin having both a radiation curable ethylenically unsaturated double bond and a salt group. A coating film thereof is cured by radiation, and the solvent resistance is enhanced as the acryloyl group concentration and crosslinking degree increase.
However, the adhesion and the workability tend to be deteriorated as the crosslinking degree increases. A salt group and a polar group must be introduced to obtain an aqueous resin composition. Therefore, even if the crosslinking degree of the composition is increased, it becomes difficult to obtain sufficient solvent resistance to a polar solvent.
If the crosslinking degree of the binder resin is drastically increased to obtain the desired solvent resistance, the coating film becomes brittle. Therefore, a metal sheet having such a cured coating film cannot be used for purposes that require high workability which can endure drawing, and that require adhesion.
Japanese Unexamined Patent Application, First Publication No. Hei 10-298213 discloses, as a composition having excellent stability of an emulsion and good adhesion to a base material, a radiation curable emulsion containing a radiation curable emulsifier which has a polymerizable double bond and also has an anionic hydrophilic group such as a carboxyl group, phosphoric acid ester group, carboxylic acid salt group, phosphoric acid ester salt group, or the like.
However, it is difficult for the resulting emulsion type composition to be homogenized during the formation of a coating film thus resulting in poor physical properties, such as surface smoothness and gloss of the coating film, as well as poor solvent resistance and poor chemical resistance.
Since the emulsion composition exhibits non-Newtonian flow, the resulting coating composition is inferior in coatability during roll coating as compared to a homogeneous water-soluble type coating composition.
Such a drawback that the emulsion composition is not homogenized during the formation of the coating film arises particularly in a thin cured coating film having a thickness of several microns, thus resulting in poor solvent resistance of the coating film, poor adhesion to a base material, and poor workability.
Japanese Unexamined Patent Application, First Publication No. Sho 61-83262, Japanese Examined Patent Application, Second Publication No. Hei 2-41555 and Japanese Unexamined Patent Application, First Publication No. Hei 5-320568 disclose a thermosetting composition, the adhesion of which is improved by adding a reaction product of an epoxy resin and phosphoric acid, or an epoxy resin, phosphoric acid ester, and carboxylic acid to an aqueous acrylic resin and an aqueous amino resin.
Although these compositions have excellent adhesion, the thin coating film having a thickness of several microns made from these compositions is inferior in solvent resistance, adhesion, and workability.
As described above, although various attempts have been made, a coating film formed on the metal sheet has not been known which has excellent adhesion to the surface of the metal sheet and maintains coating film adhesion while enduring severe working such as deep drawing, and which has excellent corrosion resistance and exhibits very high durability to a polar solvent and an alkali chemical solution.